Tracking solar collector with non-uniform solar cells and empirical tracking system including solar angle information

ABSTRACT

A tracking concentrator with a positioning system for alternately aiming the tracking solar concentrator based on the sun or a database is disclosed. The preferred positioning system comprises a sensor adapted to detect an incident light level; a tracking database comprising solar angle information; an orientation processor; one or more actuators for aiming the one or more optical elements based on the orientation processor; wherein the orientation processor is configured to track the sun based on (a) the receiver if the sensed light level exceeds a determined threshold, and (b) the tracking database if the sensed light level does not exceed the determined threshold.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims the benefit of U.S. Provisional PatentApplication Ser. No. 60/788,543 filed Mar. 30, 2006, entitled“SELF-ORIENTING SOLAR COLLECTOR WITH NON-UNIFORM SOLAR CELLS ANDEMPIRICAL TRACKING SYSTEM,” which is hereby incorporated by referenceherein for all purposes.

TECHNICAL FIELD

The invention generally relates to solar concentrators for tracking thesun. In particular, the invention relates to a concentrator adapted totrack the sun using a (1) differential current measurement from parallellines of photovoltaic cells or (2) historical tracking database,depending on the environmental conditions.

BACKGROUND

Solar concentrators employ reflectors, lenses, or a combination thereofto concentrate incident light on a receiver. The resulting irradiancepattern projected onto the receiver is generally fairly non-uniform. Theintensity in the central region of the receiver is generally muchgreater than the intensity at the periphery. This intensity variationcan significantly impact the collection efficiency where the receiverconsists of photovoltaic cells. Some concentrators use a secondaryreflector to homogenize the light from the primary reflector. Althoughthe secondary can improve the uniformity of the pattern of light on thereceiver cells, some non-uniformity generally persists and theadditional reflections in the secondary generally result in someinadvertent absorption of light. Additional loss of incident light mayresult from tracking errors that occur while the concentrator attemptsto locate the sun or re-locate the sun after it reemerges from behindcloud cover, for example. There is therefore a need for a trackingconcentrator that can compensate for non-uniform irradiance patterns,minimize losses due to tracking errors, and minimize losses do to targetacquisition or re-acquisition.

SUMMARY

The problems with the prior art are addressed with the trackingconcentrator of the preferred embodiment of the present invention, whichfeatures a positioning system for aiming a tracking solar concentratoror solar collector having one or more optical elements and a receiver.The preferred reflector positioning system comprises a sensor adapted todetect an incident light level; a tracking database comprising solarangle information; an orientation processor; one or more actuators foraiming the one or more optical elements based on the orientationprocessor; wherein the orientation processor is configured to track thesun based on (a) the receiver if the sensed light level exceeds adetermined threshold, and (b) the tracking database if the sensed lightlevel does not exceed the determined threshold.

In some embodiments, the solar angle information includes elevationangles and/or azimuth angles from a previous day. For example, the solarangle information used for tracking on a particular day may correspondto elevation/azimuth angles acquired while tracking during the one ormore days preceding the particular day. The threshold that governs, atleast in part, whether the tracking concentrator uses the trackingdatabase is a user-defined threshold selected such that the orientationprocessor tracks the sun based on the tracking database when incidentlight is obscured by substantial cloud cover.

In some embodiments, the receiver comprises one or more photovoltaiccells configured in two substantially parallel arrays with the cells ofeach array being electrically connected in series. The orientationprocessor may then be configured to track the sun based, at least inpart, on a current differential between the two parallel arrays ofcells. In addition, the surface area of the photovoltaic may be selectedsuch that each of the cells receives a substantial equal lightintensity, thereby compensating for non-uniformity of the irradiancepattern projected onto the receiver. In particular, at least two of theplurality of photovoltaic cells may possess substantially differentsurface areas.

An exemplary method of tracking the sun with the solar concentrator ofthe preferred embodiment comprises receiving a light level signal from asensor during daylight hours; tracking the sun based on (a) the sensor,if the sensed light level exceeds a determined threshold; and (b) thetracking database comprising solar angle information, if the sensedlight level does not exceed the determined threshold; and driving one ormore actuators for aiming the one or more optical elements based on thesensor or the tracking database. As described above, the receiver mayinclude photovoltaic cells configured in two substantially linear arrayswith the cells of each array electrically connected in series.Similarly, the photovoltaic cells may have a surface area that isselected such that each of the cells receives substantial equal lightintensity when tracking the sun.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention is illustrated by way of example and notlimitation in the figures of the accompanying drawings, and in which:

FIG. 1 is a diagrammatic side view of a tracking solar energyconcentrator, in accordance with the preferred embodiment of the presentinvention;

FIGS. 2A and 2B are of a receiver with two parallel lines ofphotovoltaic cells for generating a tracking signal, in accordance withthe preferred embodiment of the present invention;

FIG. 3 is a plan view of a receiver with a plurality of equally-sizedphotovoltaic cells used in some prior art solar concentrators;

FIG. 4A is a non-uniform irradiance profile projected onto the receiverin some solar concentrators;

FIG. 4B is the current profile acquired with equally-sized photovoltaiccells and a non-uniform irradiance pattern;

FIG. 5 is a plan view of a receiver with a plurality ofdifferently-sized photovoltaic cells, in accordance with the preferredembodiment of the present invention;

FIG. 6A is a non-uniform irradiance profile projected onto the receiverwith a plurality of differently-sized photovoltaic cells;

FIG. 6B is the current profile acquired with differently-sizedphotovoltaic cells and a non-uniform irradiance pattern, in accordancewith the preferred embodiment of the present invention;

FIG. 7 is a historical tracking database compiled by the trackingconcentrator, in accordance with the preferred embodiment of the presentinvention;

FIG. 8A is a plot of the daily azimuth angle data retained in thetracking database, in accordance with the preferred embodiment of thepresent invention; and

FIG. 8B is a plot of the daily elevation angle data retained in thetracking database, in accordance with the preferred embodiment of thepresent invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Illustrated in FIG. 1 is a schematic side view of a tracking solarconcentrator. The solar concentrator 100 comprises one or morereflectors 102, a receiver 104, and a reflector positioning system 106with an orientation processor 108 for orienting the concentrator 100towards the radiation source. The reflector positioning system 106includes one or more actuators adapted to aim the reflector 102 inaccordance with the orientation processor 108, thereby aiming thereflectors 102 to collect the incident radiation 120 and direct thereflected rays 122 to the receiver 104 as the radiation source moves. Inthe preferred embodiment, the reflector 102 is a parabolic trough, forexample, although various other optical elements may also be employedincluding reflectors with cylindrical, parabolic, or faceted profile, aswell as lenses with standard or Fresnel convergent lens. The receiver104 preferably includes photovoltaic cells arrayed to form a line focusreceiver adapted to receive solar flux along a line orientedapproximately perpendicular to the page. Based on the sensed currentacquired by the receiver 104, the orientation processor 108 generatesone or more tracking or alignment-error signals to re-orient thereflector 102 and maximize the collection of incident radiation 120. Theone or more tracking signals are used to drive the concentrator in aclosed-loop control mode, which is the default mode enabled when the sunis visible and there are no adverse weather conditions or other forms ofinterference that obstruct the radiation source.

Illustrated in FIG. 2A and FIG. 2B is an exemplary receiver employed inthe solar concentrator. The receiver 102 includes a plurality of energysensors, preferably photovoltaic cells 202 for converting the receivedrays into electricity, although one skilled in the art will appreciatethat various other forms of energy converters may also be employedincluding thermal absorbing liquids, heat exchangers, heat engines, andturbines, for example. The plurality of photovoltaic cells C₁-C₈ 202 arearrayed two dimensionally along the length of the line focus receiver104 symmetrically disposed about the centerline 208 of the receiver. Thecenterline 208 coincides with or is in proximity to a focal linetraversing the length of the solar concentrator parallel to theparabolic trough. Although the cells 202 are illustrated with a gapinterposed between the cells to demonstrate that one or more cells oneither side of the receiver centerline 208 may be electrically coupledin series via conductors 204, one skilled in the art will appreciatethat the gap is generally minimized or eliminated in order to maximizethe collection of light.

In the preferred embodiment, a first set of one or more cells C₁-C₄ tothe left of the centerline 208 are coupled in series with a firstammeter 208, for example, while a second set of one or more cells C₅-C₈to the right of the centerline 208 are coupled in series with a secondammeter 210. The light impinging on the receiver 104 creates anillumination footprint represented by outline 220. When the solarconcentrator 100 is precisely oriented toward the sun, the first set ofcells C₁-C₄ is illuminated with the same solar flux as the second set ofcells C₅-C₈. The receiver 104 is therefore balanced and the current I₁received from the first set of cells C₁-C₄ is substantially equal tothat of the current I₂ received from the second set of cells C₅-C₈,thereby indicating that the concentrator is properly focused on the sun.

As illustrated in FIG. 2B, the illumination footprint or outline 222 ofthe impinging light shifts approximately perpendicular to the centerline208 as the Earth rotates and the sun traces its diurnal path across thesky. With the light now improperly directed on the receiver 104, ahigher current is sensed in the first set of cells C₁-C₄ than the secondset of cells C₅-C₈. The orientation processor 108 then generates atracking signal proportional to the difference in current between I₁from the first ammeter 208 and I₂ from second ammeter 210. The trackingsignal is provided as feedback to the reflector positioning system 106.The reflector positioning system 106 adjusts the orientation of thereflector 102 via one or more actuators to minimize the tracking error,thereby maximizing the absorption of solar flux and balancing the solarflux impinging on first and second sets of cells. The tracking signal iscontinuously monitored for purposes of tracking the sun.

Unlike prior art systems, the preferred embodiment of the presentinvention permits one to directly measure and control the output power,for example, in real time, without the need for dithering commonlyemployed in concentrators having a single string of solar cells that arewired in series. Dithering is generally less efficient because itrequires that the concentrator be deliberately oriented away from theideal track point in order to estimate the location of the ideal trackpoint.

Illustrated in FIG. 3 is a multi-cell receiver consistent with the somemodern solar concentrators. The receiver 300 includes a plurality ofcells arrayed two dimensionally about the centerline 310 of thereflector 102 such as a parabolic trough, for example. In contemporaryimplementation, the centerline 310 coincides with or is in proximity tothe focal line of the reflector 102. Subsets of two or more cells of theplurality of receiver cells may be operatively coupled in series.

Due to systematic errors in the concentrator and the non-zero diameterof the solar disk, for example, the distribution of light across thereceiver 300 is generally characterized by a non-uniform intensityprofile. As is schematically illustrated in FIG. 4A, the solar fluxintensity varies across the width of the receiver perpendicular to thecenterline 310. The intensity in the central region 410 of the receiver310 coinciding with cell C₂₂ and cell C₂₃ is generally maximal while theintensity in proximity to the periphery 420 at cells C₂₁ and cell C₂₄ issignificantly less than maximal. As illustrated in the current flowdiagram of FIG. 4B, the current induced in the cells varies depending onwhether the cell lies in the central region 410 or the periphery 420. Inthis example, cell C₂₁ and cell C₂₄ generate significantly less currentthan the maximal current generated by cell C₂₂ or cell C₂₃. The fluxnon-uniformity may also occur in cells arrayed in the direction parallelto the centerline 310 as well.

The non-uniform flux intensity, unfortunately, may significantly impactthe efficiency of the receiver 300. Where a plurality of the cells areelectrically coupled in series, the maximum current flow through thestring of cells is limited to the current induced in the cell generatingthe least power. If cells C₂₁, C₂₂, C₂₃, and C₂₄ were coupled in series,the current passing through cells C₂₂ and C₂₃ would be limited to thecurrent generated by cells C₂₁ and C₂₄.

To overcome the limitations of the prior art and enable photovoltaiccells to be combined in series without the loss of efficiencyexperienced in the prior art, the solar energy concentrator 100 in someembodiments employs a plurality of cells that are sized to receive thesame solar flux—an equal number of photons per cell—in the presence ofthe systematic errors with respect to the illumination of the receiverby the concentrator. In particular, the area of the cells is varied tocompensate for the relative differences in solar flux that may beacquired by cells when the concentrator is perfectly aimed toward theradiation source. As illustrated in FIG. 5, the cells in the region ofmaximal solar flux, represented by cell C_(A), have a relatively smallsurface area while the cells in the penumbra, represented by cell C_(B)and cell C_(C), have a relatively large surface area. The cells C_(B),C_(C) in the penumbra, however, are adapted to acquire the same totalflux and generate the same current as the cell C_(A) and cell C_(F) inthe central region of the receiver 500. The solar flux represented bythe area under the flux curve in FIG. 6A is equal for each of theplurality of cells C_(A)-C_(F). The current generated in the presence ofthe flux non-uniformity is also equal for each of the plurality of cellsC_(A)-C_(F). The resulting cells may then be coupled in series withoutthe cells at the periphery serving to limit the current of the cells inthe central region, thereby preventing the losses due to systematic fluxnon-uniformity.

The primary cause of flux non-uniformity are systematic errors, i.e.,errors that are predictable and repeatable. Systematic errors may occur,for example, where the intensity of a mirror reflection falls off at itsedge in a predictable way due to the non-zero diameter of the solardisk. Using the preferred embodiment of the present invention, the sizeof the cells are designed or otherwise selected to compensate forsystematic errors and equalize the current flowing such that each cellreceives the same amount of light, even though the cells are ofdifferent sizes.

In some embodiments of the present invention, the solar concentrator 100is adapted to compile a historical tracking database. This trackingdatabase includes solar direction data used to determine the position ofthe sun under adverse weather conditions, for example. The trackingdatabase provides an empirical model with which to aim the solar energyconcentrator 100, as opposed to a pre-determined astrophysical modelwhich must generally be configured upon installation. When the level ofincident light striking the receiver or other tracking sensor is below auser-determined threshold—coinciding with intervals of cloud cover—theorientation processor 108 causes the reflector positioning system 106 toswitch over from the closed-loop control mode discussed above to anopen-loop control mode in which the concentrator 100 is aimed using thehistorical tracking database without the tracking signal or otherfeedback. The open-loop control mode may also be enabled at sunrise toaim the concentrator in the direction of the sun before the sun mightotherwise be located using the closed-loop control mode.

Illustrated in FIG. 7 is an exemplary historical tracking databasecompiled by the reflector positioning system 106 in the open-loopcontrol mode. The tracking database is preferably a table 700 populatedwith empirical solar angle information recorded as a function of timeduring the solar tracking operations of one or more preceding days. Thesolar angle information preferably includes the azimuth angles A₁-A_(N)and elevation angles E₁-E_(N) for times T₁-T_(N), respectively, recordedbetween sunrise and sunset. The tracking database 700 of FIG. 7 isillustrated graphically in the daily azimuth angle plot of FIG. 8A andthe daily elevation angle plot of FIG. 8B. In the preferred embodiment,the historical tracking database is used on when a fail-over conditionoccurs, i.e., when the view of the sun in compromised by a cloud orother ubstruction. When the fail-over condition is satisfied and theopen-loop control mode is enabled at time T_(n), the reflectorpositioning system 106 retrieves the associated azimuth angle A_(n) andelevation angle E_(n) and direct the reflectors accordingly. Thereflector positioning system 106 continues to orient the reflector 102in accordance with the solar angle information as long as the fail-overcondition is satisfied or until sunset.

In the preferred embodiment, the solar angle information includesreference data compiled from one or more previous days. By default, thereflector positioning system 106 uses the solar angle informationrecorded from the previous day. If, however, the previous day was also acloudy day, for example, the data from two days prior is used. Ingeneral, the solar angle information may be associated with the lastrecorded day for which no fail-over condition occurred. Solar angleinformation collected and recorded during a given day may be discardedand not incorporated into the historical tracking database if the datacollected is corrupted or compromised, i.e., one or more fail-overconditions were detected during its acquisition. In some embodiments,the decision whether to replace one or more historical solar angle datapoints with current angle data associated with the same point in time,i.e., same time stamp, may occur in real-time provided the fail-overcondition is not satisfied.

The temporal resolution of the time data T₁-T_(N)—which may be on theorder of one or more seconds or one or more minutes—is sufficient topoint the concentrator 100 in the direction of the sun for the durationof the adverse weather condition. In the preferred embodiment, thereflector positioning system 106 generates a time of day estimate usinga clock that is set by associating the time that the sun was highest inthe sky with local noon, thereby obviating the need for a precisionclock or burdensome user configuration. One skilled in the art willappreciate that the third preferred embodiment may be used to track thesun on a partly cloudy day or days where the sun is not continuouslyvisible. While prior art concentrators typically lose time re-acquiringtracking when the sun comes back out, the present invention is able torapidly re-acquire the target and reduce the power loss that mightotherwise occur as the system slews to the correct tracking positionwhen the sun comes out from behind a cloud, for example. As anadditional advantage, the preferred embodiment of the present inventionis unaffected by the way the unit is oriented on the user's roof andrequires no calibration at time of installation or thereafter. Even whenthe concentrator 100 is tilted or rotated in arbitrary way, theconcentrator automatically continues to track the sun in as little asone day since the empirical model is based on conditions observed theprevious one or more days. It also requires no complex mathematics oralgorithm development, and does not require that the concentrator 100 beaware of the actual local time since the local time is simply inferredbased on the track of the sun across the sky.

One or more embodiments of the present invention may be implemented withone or more computer readable media, wherein each medium may beconfigured to include thereon data or computer executable instructionsfor manipulating data. The computer executable instructions include datastructures, objects, programs, routines, or other program modules thatmay be accessed by a processing system, such as one associated with ageneral-purpose computer or processor capable of performing variousdifferent functions or one associated with a special-purpose computercapable of performing a limited number of functions. Computer executableinstructions cause the processing system to perform a particularfunction or group of functions and are examples of program code meansfor implementing steps for methods disclosed herein. Furthermore, aparticular sequence of the executable instructions provides an exampleof corresponding acts that may be used to implement such steps. Examplesof computer readable media include random-access memory (“RAM”),read-only memory (“ROM”), programmable read-only memory (“PROM”),erasable programmable read-only memory (“EPROM”), electrically erasableprogrammable read-only memory (“EEPROM”), compact disk read-only memory(“CD-ROM”), or any other device or component that is capable ofproviding data or executable instructions that may be accessed by aprocessing system. Examples of mass storage devices incorporatingcomputer readable media include hard disk drives, magnetic dish drives,tape drives, optical disk drives, and solid state memory chips, forexample. The term processor as used herein refers to a number ofprocessing devices including general purpose computers, special purposecomputers, application-specific integrated circuit (ASIC), anddigital/analog circuits with discrete components, for example.

Although the description above contains many specifications, theseshould not be construed as limiting the scope of the invention but asmerely providing illustrations of some of the presently preferredembodiments of this invention.

Therefore, the invention has been disclosed by way of example and notlimitation, and reference should be made to the following claims todetermine the scope of the present invention.

1. A reflector positioning system for a tracking solar concentratorcomprising one or more optical elements and a receiver, the reflectorpositioning system comprises: a sensor adapted to detect an incidentlight level; a tracking database comprising solar angle information; anorientation processor; one or more actuators for aiming the one or moreoptical elements based on the orientation processor; wherein theorientation processor is configured to track the sun based on: a) thereceiver if the sensed light level exceeds a determined threshold; andb) the tracking database if the sensed light level does not exceed thedetermined threshold.
 2. The positioning system of claim 1, wherein thesolar angle information comprises elevation angles and azimuth angles.3. The positioning system of claim 2, wherein the solar angleinformation used for tracking on a particular day includes the trackingangles acquired while tracking on the one or more days preceding theparticular day.
 4. The positioning system of claim 1, wherein thedetermined threshold is user-defined.
 5. The positioning system of claim4, wherein the threshold is selected such that the orientation processortracks the sun based on the tracking database when incident light isobscured by substantial cloud cover.
 6. The positioning system of claim1, wherein the receiver comprises one or more photovoltaic cells.
 7. Thepositioning system of claim 6, wherein the one or more photovoltaiccells of said receiver are configured in two substantially parallelarrays, the cells of each array being electrically connected in series.8. The positioning system of claim 7, wherein the orientation processoris configured to track the sun based at least in part on a currentdifferential between the two arrays of cells.
 9. The positioning systemof claim 7, wherein the photovoltaic cells have a surface area selectedsuch that each of the cells receives a substantial equal lightintensity.
 10. The positioning system of claim 9, wherein at least twoof the plurality of photovoltaic cells possess substantially differentsurface areas.
 11. The positioning system of claim 1, wherein the one ormore optical elements comprise: one or more reflectors with acylindrical, parabolic, or faceted profile; or one or more convergentlenses; or one or more Fresnel lenses.
 12. The positioning system ofclaim 2, wherein the elevation angles and azimuth angles are recorded asa function of time, wherein the time is derived clock that is set byequating the highest position of the sun with noon local time.
 13. Amethod of tracking the sun with a solar concentrator comprising one ormore optical elements and a receiver, the method comprising: receiving alight level signal from a sensor during daylight hours; tracking the sunbased on: a) the sensor, if the sensed light level exceeds a determinedthreshold; and b) a tracking database comprising solar angleinformation, if the sensed light level does not exceed the determinedthreshold; and driving one or more actuators for aiming the one or moreoptical elements based on the sensor or the tracking database.
 14. Themethod system of claim 13, wherein the one or more photovoltaic cellscomprise said receiver.
 15. The method of claim 14, wherein the one ormore photovoltaic cells of said receiver are configured in twosubstantially linear arrays, wherein the cells of each array areelectrically connected in series.
 16. The method of claim 15, whereinthe photovoltaic cells have a surface area that is selected such thateach of the cells receives a substantial equal light intensity whentracking the sun.
 17. The method of claim 16, wherein at least two ofthe plurality of photovoltaic cells possess substantially differentsurface areas.